(1) Field of the Invention
The present invention relates to a liquid crystal display device and an electroluminescent (EL) display device, which are applicable to so-called mobile information devices or the like. The invention also relates to methods of driving the display devices. More particularly, the invention relates to a liquid crystal display device and an EL display device in which halftone display is performed by a spatial dithering method, and to methods of driving the devices and of evaluating subpixel arrangement patterns.
(2) Description of the Prior Art
A conventional pixel circuit configuration for an active matrix liquid crystal display panel is shown in FIG. 39 (hereafter, the example of active matrix liquid crystal display panel depicted in FIG. 39 is referred to as “the first prior-art example”). FIG. 39 shows source lines (data lines) SL, gate lines (scanning lines) GL, a pixel transistor Tr, a storage capacitor C, a pixel electrode 155, and a storage capacitor line 156. Insofar as the input image data signal for obtaining gray scale display has an analog signal format, this pixel configuration can attain display images of good quality. Recent mobile information devices, however, utilize digital signal formats for the image data, and therefore, when the configuration of the first prior-art example is adopted for driver circuits of the mobile information devices, digital-to-analog converter circuits are required to convert digital image input signals to analog signals. This incurs high manufacturing cost and considerable increase in power consumption.
In view of such a problem in the first prior-art example, Japanese Unexamined Patent Publication No. 10-68931 discloses a technique in which a unit pixel is divided into a plurality of subpixels and spatial dithering is employed for displaying images using digital image data signals (this example is hereafter referred to as “second prior-art example”). This prior-art technique is detailed below referring to FIG. 40. FIG. 40 shows source lines SL, gate lines GL, pixel transistors Tr1, Tr2, Tr3, and Tr4, and pixel electrodes 155a, 155b, 155c, and 155d. The pixel electrodes 155a, 155b, 155c, and 155d are formed so that the area ratio of the pixel electrodes corresponds to the weight ratio of the corresponding binary bits of digital image data. This configuration eliminates the need for converting digital image input signals to analog signals, and therefore digital signals can be used for achieving gray scale display. Thus, the configuration makes it possible to prevent increases in manufacturing cost and in power consumption which are caused by providing digital-to-analog converter circuits.
However, the second prior-art example has the following problems:
(1) In the second prior-art example, both the scan driver circuit and the data driver circuit are formed of polycrystalline silicon. For this reason, when the number of pixels increases, that is, when the horizontal scanning time shortens as the number of pixels increases, the data driver circuit does not operate at a sufficient speed because the characteristics of polycrystalline silicon are inferior to those of single crystalline silicon.
(2) In the second prior-art example, in the case where each unit pixel is divided into a plurality of subpixels, the wiring configuration of data lines and scanning lines is such that the data lines are individually wired and the scanning lines are commonly wired. This structure significantly increases the number of connecting pins in the driver circuits, especially when each unit pixel needs to be further divided for R (red), G (green), and B (blue) colors to attain color image display. As a result, the incidence of connection defects increases, resulting in image quality degradation which include display defects.
(3) In the second prior-art example, the power consumption can be reduced by performing gray scale display with the use of digital signals. However, further power reduction is necessary when the second prior-art example is desired to be used for information terminal devices, particularly for displays in mobile telephones.
(4) In the second prior-art example, the pixel transistors Tr1, Tr2, Tr3, and Tr4 drive the pixel electrode 155a, 155b, 155c, and 155d, respectively, so as to perform gray scale display. Consequently, depending on the relationship between the gray level in a certain unit pixel and that in an adjacent unit pixel, stripe-like shaded patterns (persistent patterns) appear on the display screen, degrading image quality of the liquid crystal panel.